1. Field of the Invention
The present invention relates to an inkjet recording apparatus and an inkjet recording method, and in particular, relates to an inkjet recording apparatus and an inkjet recording method that inhibit quality degradation caused by a change in discharge quantity of ink.
2. Description of the Related Art
In recent years, higher performance of a recording apparatus used in a printer, copying machine, facsimile and the like is increasingly demanded and not only fast recording/full-color recording, but also high-resolution image recording equivalent to a silver halide photography is called for. Confronted with such demands, an inkjet recording apparatus is superior in fast recording and high-quality recording to recording apparatuses that adopt other recording methods because the inkjet recording apparatus can discharge fine ink droplets at a high frequency. Particularly, the method of using bubbles generated by an electrothermal converter (hereinafter, referred to as a heater) like a heater element as a means for discharging ink droplets, the so-called thermal inkjet recording method (for example, Japanese Patent Publication No. 61-59911) can as its feature make miniaturization of an apparatus easy and create higher-density images.
The thermal inkjet recording method is a method by which an electric signal (hereinafter, referred to as a pulse) is applied to a heater to be converted into thermal energy in an inkjet recording head (hereinafter, referred to a head or a recording head), the thermal energy is used to cause ink to film-boil, and pressure of bubbles caused by the boiling is used to cause the ink to discharge so that discharged ink droplets adhere to a recording medium and dots are formed to output an image in the end.
According to the thermal inkjet recording method, the discharge quantity of ink is known to vary depending on viscosity of the ink. The viscosity of ink varies widely depending on the temperature. The discharge quantity of ink varies depending on the temperature of ink near the heater. Because the viscosity of ink increases as a temperature decreases, thereby reducing fluidity of ink and growth of bubbles caused by film boiling is less promoted with a decreasing temperature, the discharge quantity of ink varies due to variations in temperature of ink near the heater. Thus, due to a head temperature rise caused by heating of the heater resulting from printing, the discharge quantity of ink increases compared with that before the head temperature rises.
Therefore, if the head temperature changes during recording such as printing, particularly in recording an image like photograph, a change in density occurs in an output image. As a result, unevenness in density may arise in a recorded image, causing degradation in recording quality. This conspicuously appears due to a dramatic change of the head temperature when the heater is driven at a higher frequency or the number of discharge ports is increased to lengthen the line length of the discharge ports array to realize fast recording demanded in recent years.
As described above, a problem in achieving a faster recording speed is image degradation originating from variations in discharge quantity of ink due to a temperature rise of the recording head. Thus, to obtain an excellent image, various kinds of control to stabilize the discharge quantity of ink discharged from the recording head have been performed for the purpose of inhibiting an occurrence of unevenness in density in a recorded image as much as possible (see Japanese Patent Publication No. 61-59913 and Japanese Patent Publication No. 61-59914).
In the inkjet recording method in which ink is rapidly heated by applying a pulse to a heat element to cause a change of state of the ink from the liquid phase to the gas phase to generate a boiling force, the discharge quantity is almost determined by the input condition of energy during the change of state from the liquid phase to the gas phase. Thus, after the ink changes to the gas phase, the discharge quantity is hardly affected no matter how energy is input.
One conventional measure against variations in discharge quantity originating from a temperature rise of the head in an inkjet recording apparatus is to control the input condition of energy until the state of the ink changes to the gas phase. For example, a time chart of the pulse to be applied to the recording head is illustrated in FIG. 9 as an example of the condition. A known method modulates the discharge quantity by using a divided pulse (double pulse) as illustrated in FIG. 9 as a pulse applied to the recording head and controls a pre-pulse, a main pulse, and an interval time between these pulses.
The pulse width of a pulse to drive the recording head and a drive voltage Vop are determined by the area of a heater board where the heater is arranged, heater resistance, film structure of the heater board, or structure of a nozzle formed by discharge ports and flow paths of the recording head.
Based on temperature information from a temperature detecting element (hereinafter, described as a temperature sensor) provided in the recording head, the waveform of at least one pulse of a pre-pulse P1, an interval time P2, and a main pulse P3 is modulated. In FIG. 9, times T1, T2, and T3 are a rise time or fall time of the applied pulse and indicate times to determine pulse widths of the pre-pulse P1, the interval time P2, and the main pulse P3 respectively.
The pre-pulse P1 is the pulse mainly to control the temperature of ink in the nozzle and the pulse width thereof is controlled according to the temperature detected by using the temperature sensor of the recording head. At this time, the pulse width thereof is controlled such that ink does not boil by the pre-pulse with too much thermal energy applied to the ink.
The interval time P2 is provided for the purpose of preventing interference between the pre-pulse P1 and the main pulse P3 and also for the purpose of making the temperature of the ink inside the nozzle uniform by diffusing thermal energy provided in the pre-pulse P1 into the ink from the heater.
The main pulse P3 provides energy to boil the ink to discharge ink droplets from the discharge port.
The discharge quantity of ink can be stabilized by adjusting the pulse width of the pre-pulse P1, the pulse width of the main pulse P3, and the interval time P2, which is the interval between these pulses. If, for example, the temperature of the recording head is low and the discharge quantity of ink decreases, the pulse width of the pre-pulse P1 is adjusted to a relatively broad width. Conversely, if the temperature of the recording head is high and the discharge quantity of ink increases, the pulse width of the pre-pulse P1 is adjusted to a relatively narrow width.
Thus, variations in discharge quantity can be inhibited by modifying the waveform of pulse based on the head temperature detected by the temperature sensor or the like.
To inhibit variations in discharge quantity with higher precision, various ways shown below have been devised.
As described above, the temperature of the recording head is detected by the temperature sensor provided in the recording head. Thus, if temperature detection performance of the detecting element varies, the temperature cannot be detected correctly and appropriate corrections of the discharge quantity cannot be made.
Japanese Patent Application Laid-Open No. 11-240148 discusses a method of correcting variations of the detected head temperature originating from variations of temperature detection precision of the temperature sensor provided in the recording head. According to Japanese Patent Application Laid-Open No. 11-240148, in addition to the temperature sensor provided in the recording head, a high-precision thermistor sensor is provided near the head inside the recording apparatus. The high-precision thermistor can determine approximately the correct temperature and by comparing temperatures detected by these two sensors before printing, an error of temperature measurement by the temperature sensor provided in the recording head can be checked.
As described above, corrections of the discharge quantity dealing with a change in head temperature are made by changing the pulse. If the resistance of the heater element varies, the heating value of the heater varies even if the same pulse is applied and, as a result, variations in discharge quantity may occur.
According to Japanese Patent Application Laid-Open No. 2007-69575, this problem is solved by allocating rank to each head based on the heater resistance and changing the pulse for each head based on the ranking.
As described above, it is the temperature near the heater that affects the discharge quantity of ink. On the other hand, the temperature of the recording head is detected by the temperature sensor provided in the recording head. In a common recording head, the temperature sensor is composed of a diode sensor or a thermistor provided on the heater board. The temperature sensor may be preferably arranged near each heater to directly measure the temperature near each heater, but a common inkjet head has heaters arranged densely. Therefore, the temperature sensor is structured such that one or several temperature sensors are arranged slightly apart from a region where heaters are arranged, which makes direct measurement of the temperature near each heater impossible.
Japanese Patent Application Laid-Open No. 2008-168626 discusses a method of inhibiting a discharge quantity change in a heater column by predicting from the temperature detected by the sensor the temperature distribution in the heater column in which heaters are arranged and applying an appropriate pulse to each heater. Japanese Patent Application Laid-Open No. 2008-168626 also discusses a phenomenon in which the temperature distribution in the heater column changes due to unevenness in thickness of the adhesive between the heater board and a member to which the heater board is bonded.
Japanese Patent No. 03530843 discusses a phenomenon in which the temperature detected by a temperature detection diode varies when the same signal is applied to a heater depending on the connection state between the heater board and a member to which the heater board is bonded. Japanese Patent No. 03530843 utilizes this phenomenon for non-discharge detection (detection that discharge is not executed) in consideration of variations of the connection state of the heads.